The present invention relates to the field of coal-fired furnaces, and more particularly, to pulverized coal-fired furnaces designed as direct-fired systems. Specifically, this invention is directed to an apparatus for supplying pulverized coal simultaneously to the direct-fired furnace and to a storage bin from one pulverizer.
In order to avoid the high cost of oil and gas, electric utilities have increasingly chosen coal as the fuel to fire the furnaces of their steam generating boilers. However, even in coal-fired furnaces, substantial quantities of oil and gas are often used. In a typical coal fired unit, the coal must be dried and pulverized in a pulverizer by heated air before it can be burned in the furnace. The heated air used to dry the coal is supplied by a forced-draft fan that forces the air through a preheater wherein the air is passed in heat exchange with hot combustion products leaving the furnace.
Therefore, it is necessary that the furnace be already operating in order to dry the coal for the coal to be burned in the furnace. Accordingly, in a typical pulverized coal-fired furnace, a relatively large capacity oil burner is started by an ignitor and operated for a fairly long period of time to warm up the furance walls and the heat exchange surfaces of the air preheater. Once the furnace has been brought up to temperature, the pulverized coal is supplied to the furnace and ignited by oil or gas ignitors associated with the coal burners.
It has been determined that the use of auxiliary fuels such as oil or gas can be minimized by warming the furnace up on pulverized coal which has been pulverized and dried previously when the furnace was in operation and stored in the interim in a storage bin. When it is necessary to warm the furnace up, the pulverized coal is fed to the furnace from the storage bin, typically in a dense phase stream, and ignited in the furnace by a small oil or gas ignitor. Additionally, it has even been suggested that the oil and gas ignitors can be eliminated by using a coal-fired ignitor supplied with pulverized coal from storage bin lit off by the use of an electric spark plug. U.S. Pat. No. 4,173,189 discloses one method and apparatus for using pulverized coal in ignitor burners for the cold start-up, warm-up, and low load stabilization of a pulverized coal-fired furnace wherein the pulverized coal for start-up and warm-up is supplied from a storage bin.
Additionally, it has been proposed that pulverized coal from a storge bin be used in conjunction with a direct-fired furnace in order to increase load capacity on the furnace. One method for utilizing supplemental pulverized coal from a storage bin for increasing load capacity is disclosed in U.S. Pat. No. 4,263,856. Therein, it is disclosed that pulverized coal from the storage bin may be conveyed as a dense phase mixture of pulverized coal and air having a air to coal rate ratio below approximately 1.0 and injected into the main pulverized coal stream being supplied from the pulverizers in order to increase the capacity of the pulverizers.
In either of these systems for utilizing pulverized coal from a storage bin, it is customary to supply pulverized coal to the storage bin from either a pulverizer set aside only for that purpose or from one of the load carrying pulverizers of the direct-fired furnace when that pulverizer is not needed to carry load on the furnace. However, there are times when it will be desirable to use a load carrying pulverizer of the direct-fired furnace simultaneously for both maintaining load on the furnace and supplying pulverized coal to the storage bin.